1. Field of the Invention
The present invention relates to a light scanning device, and an image forming apparatus using the light scanning device.
2. Description of the Related Art
A light flux from a light source is deflected by light deflecting means such as a rotational polygon mirror. The deflected light flux is focused to a scanned surface by using a scanning imaging optical system including an fθ lens. In this manner, a light spot is formed on the scanned surface. A light scanning device that uses the light spot to scan the scanned surface is well known as a device related to an image forming apparatus such as an optical printer, an optical plotter, and a digital copier.
In the image forming apparatus using the light scanning device, an image writing process of writing an image by light scanning is adopted as one process in an image forming operation. Quality of an image formed by the imaging process is affected by quality of light scanning. The quality of light scanning depends on scanning characteristics of a main scanning direction and a sub scanning direction.
Speed uniformity of light scanning is one of the scanning characteristics of the main scanning direction.
For example, when the rotational polygon mirror is used as the light deflecting means, a light flux is deflected in a uniform angular speed manner. For this reason, in order to realize speed uniformity of light scanning, a scanning imaging optical system that includes fθ characteristics is used.
However, since there are other characteristics required for the light imaging optical system, it is difficult to completely realize the fθ characteristics. Accordingly, in actual light scanning, there are no cases where the light scanning is performed in a completely uniform angular speed, so that the speed uniformity as the scanning characteristics is different from ideal light scanning speed uniformity.
The scanning characteristics of the sub scanning direction include a scanning line curve and a scanning line inclination.
A scanning line is a trajectory of a light spot on the scanned surface, and an ideal scanning line is a straight line. The light scanning device is designed such that the scanning line becomes a straight line. However, in reality, scanning line curve is usually generated due to a machining error, and an assembling error.
Further, in a case where an imaging mirror is used in the scanning imaging optical system, and the deflected light flux is made to have an angle in terms of the sub scanning direction of the deflected light flux between an incident direction and a reflection direction of the deflected light flux to the imaging mirror, in principle, the scanning line curve is generated. Even when the scanning imaging optical system is configured as a lens system, it is impossible to avoid the scanning line curve in a multi-beam scanning method in which scanning is performed on the scanned surface by using a plurality of light spots divided in the sub scanning direction.
The scanning line inclination is a phenomenon in which the scanning line does not appropriately cross the sub scanning direction at a right angle. Since the scanning line inclination is one kind of the scanning line curve, in the following description, the term “scanning line curve” may include the scanning line inclination.
When the light scanning speed is incomplete, distortion in the main scanning direction is generated in the formed image. The scanning line curve causes distortion in the sub scanning direction to be generated in the formed image.
In a case where an image is mono-color, that is, the image is formed by a single light scanning device, if the scanning line curve and incompleteness of the speed uniformity are suppressed to a certain degree, the formed image does not include distortion that a human eye can recognize. However, it is preferable that there is less distortion in the image.
Different from the mono-color image, in a conventional color copier and the like, each color component image of three colors of magenta, cyan, and yellow, or four colors of magenta, cyan, yellow, and black is formed, and these color component images are made to overlap each other in order to form a color image.
As a color image forming method, there is a tandem type image forming method in which respective color component images are formed on photoconductive bodies by using respective light scanning devices. In a case of such an image forming method, if scanning curve and inclination degrees in the respective light scanning devices are different from each other, even if the respective scanning line curves in the light scanning devices are adjusted, an abnormal image called “color difference” is generated in the formed color image, so that color image quality is deteriorated.
The color difference phenomenon includes a phenomenon in which a color tone (hue) in the formed color image is different from a desired color tone.
In order to prevent a problem of the above-mentioned color difference, Japanese Patent Application Laid Open No. 2002-131674 discloses a scanning line curve adjustment structure in which both sides of a supporting unit for a long lens is provided between an optical axis of the long lens, and one side of the supporting unit is configured such that the long lens can be moved in the sub scanning direction by an adjusting screw. In this structure, adjusting the screw causes the long lens to be rotated in a section plane orthogonal to a deflection direction to adjust the scanning line curve.
However, in Japanese Patent Application Laid Open No. 2002-131674, there is a possibility that the scanning line curve cannot be adjusted in an environment affecting a material of the lens used in the imaging optical system. The reason of this is as follows.
Recently, in order to improve scanning characteristics, a special surface of which representative is non-spherical surface was generally adopted in the imaging optical system of the light scanning device. It is relatively common to use the imaging system that enables such a special surface to be easily formed, and that is configured by a resin material of which cost is low.
The imaging optical system including the resin material can be easily affected by temperature and humidity change, resulting in change of optical characteristics. This change of the optical characteristics causes the scanning line curve degree and the speed uniformity to be also changed. For this reason, when successively forming color images of tens of pages, a temperature in an image forming apparatus is raised due to the successive operation. Accordingly, the optical characteristics of the imaging optical system are changed, and the curve of the scanning line written by the respective light scanning devices (optical writing devices), and the speed uniformity are gradually changed, in some cases, resulting in the color difference in which the color tone of the first formed image is entirely different from the color tone of the last formed image.
A scanning imaging lens such as representative fθ lens in the scanning optical system is formed as a thin lens that is long in the main scanning direction such that unnecessary portion (that the deflected light flux does not enter) of the scanning imaging lens in the sub scanning direction is cut. In a case where a plurality of lenses constitute the scanning imaging system, as the disposed position of the lens is more away from the light deflecting means, the length of the lens in the main scanning direction becomes longer, so that the long lens having the length of ten-odd cm to 20 cm or more is required. Such a long lens is generally formed in resin molding by using a resin material, but when the temperature distribution in the lens becomes inhomogeneous, the lens is warped and arched in the sub scanning direction. This warping of the long lens causes the scanning line curve, and when the warping is large, the scanning line curve also becomes very large. Even when the configuration of Japanese Patent Application Laid Open No. 2002-131674 is used to perform initial adjustment, this phenomenon is generated. Furthermore, the configuration disclosed by Japanese Patent Application Laid Open No. 2002-131674 does not take into consideration the scanning line inclination that also causes the color difference. In Japanese Patent Application Laid Open No. 2002-131674, the position of the lens in the optical axis direction is changed by screw fastening degree, so that it is difficult to maintain accuracy in the position of the lens.
Recently, accompanying higher speed color image forming, four photoconductive drums (bodies) are arranged in a direction of transporting paper, and a plurality of optical systems corresponding to these respective photoconductive drums form latent images simultaneously on these photoconductive drums. Then, developing units use developing agents of different colors (for example, yellow, magenta, cyan, and black) to turn these latent images into visible images, and these visible images are successively transferred on the same paper on top of one another to obtain a color image. An image forming apparatus (four drum tandem type) such as a digital copier and a laser printer that form a color image has been put into practical use.
In the four drum tandem type, a mono color image and a color image can be output at the same speed, so that the four drum tandem type is advantageous for high speed printing. However, when the toner images developed on the photoconductive drums are transferred to the paper on top of one another, a color difference is generated. In the case of the four drum tandem type, since a multi-color image is formed by successively transferring respective color toner images on top of one another, the generated scanning line position displacement is scattered among the respective colors, resulting in the color difference.
In order to reduce the color difference, there are the following methods.
First, Japanese Patent Application Laid-Open No. 2001-133718 discloses a method in which the position of the entirety of respective optical elements is adjusted relative to the photoconductive body so as to conform the respective scanning lines to each other. However, a mechanism for performing this adjustment becomes complicated, so that it is difficult for this method to deal with optical characteristics change due to time lapse (for example, temperature change). Furthermore, in this method, it is difficult to correct the color difference with high accuracy during a printing operation or a used state.
Secondly, Japanese Patent Application Laid-Open No. 2001-100127 discloses the method in which the scanning beam position is controlled in the sub scanning direction by using a galvanometer mirror. However, the galvanometer mirror has too high sensitivity in controlling the scanning position in the sub scanning direction. Accordingly, this method is easily affected by outside vibration. In order to realize further improved beam spot diameter, higher surface accuracy is required.
Third, Japanese Patent Application Laid-Open No. 10-239939 discloses the method in which in accordance with phase relation between a middle transfer reference signal and a line synchronization signal, out of a plurality of laser beams, a laser beam for first writing an image on a photoconductive body is switched so that an image writing start position for each color in the sub scanning direction can be adjusted. However, also in this method, it is impossible to achieve correcting accuracy of less than one line. For example, in a case of image writing of 600 dpi, a color difference (position displacement larger than 42 μm) is generated.
Further, when a tilt amount of the reflection mirror is changed, an optical path length up to the image holding body becomes different between a center part and end parts of the scanning line. This problem is called a magnification error in the main scanning direction.
The causes of the color difference, particularly, in the sub scanning direction in the tandem type image forming apparatus include: {circle around (1)} nonuniformity of speed on the photoconductive body in the circumferential direction (in the sub scanning direction); {circle around (2)} nonuniformity on the middle transfer body in the circumferential direction (in the sub scanning direction); {circle around (3)} a position error between the photoconductive bodies; {circle around (4)} writing position displacement of a beam spot between scanning optical systems; and {circle around (5)} position displacement due to {circle around (1)} to {circle around (4)} caused by environmental change or temperature change at the time of successive printing. In addition, when writing is performed on each photoconductive body by using multi beams, there is a possibility that a position is displaced by an amount depending on the number of beams because the rotation of the polygon scanner is not synchronized with the speed on the photoconductive body. Furthermore, in some cases, the color difference in the sub scanning direction is caused by change of the optical length due to an error of the thickness of the middle transfer body.